Wednesday, April 30, 2008

These images taken by NASA's Hubble Space Telescope show nine compact, ultradense galaxies as they appeared 11 billion years ago.

By SPACE.com StaffSeveral newfound galaxies seen as they existed when the universe was young are packed with improbable numbers of stars.

Astronomers don't know what's going on.

The nine galaxies are 11 billion light-years away, which means the light astronomers are looking at left the galaxies 11 billion years ago, when the universe was less than 3 billion years old.

Each of the newly studied galaxies weighs about 200 billion times the mass of the sun yet is a mere 5,000 light-years across. Our Milky Way Galaxy is a fraction of that heft at roughly 3 million times the sun's mass, and yet it stretches across 100,000 light-years of space.

The compact galaxies have been furiously forming stars; each contains as many stars as a typical large galaxy of today, the new observations reveal.

"Seeing the compact sizes of these galaxies is a puzzle," said Pieter G. van Dokkum of Yale University, who led the study. "No massive galaxy at this distance has ever been observed to be so compact."

Since no modern galaxies — galaxies in the nearby universe — are so compact, the scientists assume compact galaxies from the early universe must have gotten much larger as they matured beyond the snapshots of ancient time now being studied. But nobody knows how.

"They would have to change a lot over 11 billion years, growing five times bigger," van Dokkum said. "They could get larger by colliding with other galaxies, but such collisions may not be the complete answer."

Astronomers used NASA's Hubble Space Telescope and the W.M. Keck Observatory on Mauna Kea, Hawaii to make the new observations, which were announced today and were detailed in the April 10 issue of the Astrophysical Journal Letters.

Van Dokkum and his colleagues had previously studied the galaxies in 2006 with the Gemini South Telescope to determine their distances, and showed that the stars are a half a billion to a billion years old. The most massive stars had already exploded as supernovae.

One reason these galaxies were so dense, van Dokkum suggested, involves the interaction of dark matter and hydrogen gas in the nascent universe. Dark matter is an invisible form of matter that accounts for most of the universe's mass. Shortly after the theoretical Big Bang, the universe contained an uneven landscape of dark matter. Hydrogen gas became trapped in puddles of the invisible material, the thinking goes, and began spinning rapidly in dark matter's gravitational whirlpool, forming stars at a furious rate.

Based on the galaxies' mass, the astronomers estimated that the stars are spinning around their galactic disks at roughly 890,000 to 1 million mph (400 to 500 kilometers a second). Stars in today's galaxies, by contrast, are traveling at about half that speed because the setups are larger and rotate more slowly.

The light we receive from distant sources is generated on the tiniest of scales. To explore the largest objects, such as galaxies, we have to first understand the smallest of objects, atoms and the particles making up atoms. The photons that we detect with our eyes and catch with our telescopes were generated in many different ways: sometimes by electrons hopping between different orbital levels in an atom, or other times by the energetic collisions of atomic nuclei. We now explore the ways in which photons of light arise, how they get from there to here, and what they can tell us about the objects that we observe.We have concentrated thus far on optical photons (the ones that we can see with our eyes). As it turns out, our eyes respond to “visible” wavelengths because that is where the peak of the emission from the sun is located in the electromagnetic spectrum. If our eyes were most sensitive to infrared radiation, for example, we would see some things we can’t now see (body heat), but would miss a lot of other useful stuff. In this chapter, we’re going to talk more about visible light and the electromagnetic spectrum, of which visible light is a tiny subset. Think of it this way: If the electromagnetic spectrum is represented by a piano keyboard, then the visible part of the spectrum is but a single key or note. In the cosmic symphony, there are many notes, and we want to be able to hear them all. If you’re concerned that this sounds more like physics than astronomy, you’re right. But don’t be intimidated. Most of astronomy involves applications of physics principles, and we are convinced that understanding what you are seeing when you look at a star greatly enhances the experience of looking. Remember this astounding fact : When you look at the light from our sun or a distant star, you are witnessing the product of nuclear fusion reactions that are, every second, releasing more energy than any atomic explosion Earth has ever witnessed. Yet it is not just brute energy, but also information from the sky. Let’s take a closer look.

Next, relax. Don’t look too hard. We mean this as sincere and literal advice. Your eye’s sharpest color vision is in the center of your field of view. This is where color-receptor neurons known as cones are most densely concentrated. However, so-called rods, the visual receptors sensitive to black, white, and shades of gray, while insensitive to color, are more sensitive than cones to low levels of light. This means you can actually better see fainter objects with your peripheral vision than with your center-field vision. Learn to look askance at the stars. This practice is sometimes called “averted vision.” Using it, you will typically see fainter stars.Peering through a telescope for extended periods is fun, but it can also be fatiguing. Don’t squint. Don’t peer. Step away from your telescope periodically to walk around. Relax and enjoy.You’ll enjoy your astronomy sessions more, as well as reduce fatigue, if you practice keeping both eyes open when you look through the eyepiece. If you can’t resist the urge to close one eye, buy a pirate’s eye patch from the local toy store or costume shop. Then you can keep both eyes open without distraction and even feel like a real celestial navigator. A parrot on the shoulder is optional.

Thursday, April 24, 2008

WASHINGTON (Reuters) - Images of colliding galaxies show them spinning, sliding and slipping into one another, wreaking stellar destruction that will give birth to new and larger galaxies.

The Maryland-based Space Telescope Science Institute released 59 new images from the Hubble Space Telescope on Thursday to celebrate the 18th anniversary of its launch.

"This new Hubble atlas dramatically illustrates how galaxy collisions produce a remarkable variety of intricate structures in never-before-seen detail," the Institute said in a statement.

"Astronomers observe only one out of a million galaxies in the nearby universe in the act of colliding. However, galaxy mergers were much more common long ago when they were closer together, because the expanding universe was smaller."

The color images, available online, are a look back in time. It takes hundreds of millions of years for galaxies to merge and the light from their stars has traveled for hundreds of millions of years across space.

Because it orbits outside the Earth's atmosphere, Hubble's cameras can take extremely sharp images.

Its future was controversial, as it requires regular servicing by space shuttle astronauts to stay in working condition.

After the 2003 Columbia space shuttle disaster, a servicing mission initially planned for 2004 was canceled.

NASA at one point was planning to abandon the telescope, hugely popular among astronomers. After an outcry, the U.S. space agency relented and a final Hubble servicing mission is scheduled for August.In 2013, the James Webb Space Telescope is scheduled to replace Hubble

You have some learning to get under your belt, but right now, neophyte that you are, you can do something to enhance your experience. Unless you are looking at the bright moon, don’t rush to the eyepiece until you have allowed your vision to become “dark adapted.” This natural adjustment will greatly enhance your ability to see faint objects—and it will make brighter objects that much more exciting. Adapting your eyes to the dark requires about 15 minutes away from sources of light. If somebody shines an uncovered flashlight in your eyes, you’ll have to become dark adapted all over again. Red light, however, will not reverse dark adaptation. For those on liberal budgets, there are specially made, compact flashlights with red bulbs. For the rest of us, either equip your flashlight with a dark red filter (you can use red acetate purchased from a hobby shop) or (less effectively) simply put a red sock over the flashlight. This way, you’ll be able to see what you are doing and even consult star maps without spoiling your dark adaptation.

Understandably, you will be eager to try out your new telescope. Here are a few words of advice: Expect to be thrilled—immediately—by the spectacle of the moon, with its sharply delineated craters and mountains. Point your telescope elsewhere, however, and you may be disappointed—at least until you learn more about what to look for. We have become spoiled by dazzling images from the Hubble Space Telescope, orbiting above our atmosphere and toting the most sophisticated instruments available. No, your telescope won’t duplicate the performance of Hubble. But the point is that it is your telescope, and the photons of light that left the Orion Nebula are striking your retina. The experience is yours.

Your first impulse may be to blame any disappointment you feel on your telescope. Resist the impulse. As you learn what to look for—and as you come to appreciate the significance of what you see—you will derive great satisfaction from your instrument.

Sunday, April 20, 2008

ABC News has been given unprecedented access to the astronauts, scientists and engineers involved in the intensive — and some say risky — shuttle mission to repair the Hubble Space Telescope later this year.

Time is running out for Hubble. If its batteries and gyroscopes aren't replaced soon, the most famous of space telescopes will simply quit functioning.

If all goes well, the Space Shuttle Atlantis mission, dubbed STS 125, will launch in August to service Hubble. ABC News will follow the crew as it trains for this mission.

This will be the last time a space shuttle visits Hubble. Everyone involved knows they must make every single minute of the mission count to ensure Hubble will continue to explore the universe until the replacement Webb telescope can be launched sometime in the next decade.

What makes this mission risky?

Unlike most recent shuttle missions, this one will not be docking at the International Space Station. If Atlantis encounters an emergency, Hubble's orbit is so far from the space station, that the Atlantis crew will not be able to reach it.

But NASA is ready with an unprecedented backup plan. For the first time, when the Atlantis astronauts launch to repair Hubble, a second shuttle will already be on the other launch pad at the Kennedy Space Center, ready to go within days if its colleagues on the Hubble mission encounter a problem.

It is easy to see why the Hubble mission is the most talked about mission of the year at NASA and overshadows the global effort to build the International Space Station.

Hubble is the telescope that can look back in time; the space station is still a construction project, albeit one of the most complicated ever undertaken.

Just what kind of allure does the Hubble Space Telescope have that the International Space Station doesn't? Apollo 7 astronaut Walt Cunningham says it's all about perception.

"The International Space Station is the most incredible engineering achievement in history. It exceeds the Panama Canal or the pyramids if you will, but it doesn't capture the public's fancy, because it looks like a truck driving back and forth delivering construction materials," he said.

Hubble was deployed in 1990, and it wasn't an instant hit. Its first images were blurry because of an embarrassing failure to notice that the shape of the telescope's primary mirror was not accurate.

A daring space shuttle mission to install corrective lenses fixed that in 1993. That troubled beginning is part of its mystique, according to Sandra Faber of the University of California at Santa Cruz.

"It was such a disaster, but it was like chestnuts pulled out of the fire at the last minute. It is the ultimate American can-do story," she said.

And oh what Hubble can do! Hubble has allowed scientists to estimate the age of the universe — 14 billion years.

Faber points to Hubble's discoveries about galaxies. "First and foremost for me as [a] student of galaxy formation, Hubble was the first telescope to look back in time and show us infant galaxies, in the process of being born. That's a first. To use a telescope as a time machine looking back billions of years — that is a terrific legacy."

Matt Mountain, director of the Hubble Space Telescope, says the telescope's popularity is simple to explain.

"It allows us to see the universe in a way we don't have to explain. A picture is worth 1,000 words and so we look back in time at some of the earliest galaxies," he said.

"What we deliver back are stunning images of these fairly early galaxies," Mountain said. "And so we can't disassociate science from the image because there's actually great meaning in the science and the public actually engages in that when they look at these great pictures."

Steve Hawley is one of the astronauts who deployed Hubble. He is also an astronomer and thinks he understands why Hubble resonates with so many people.

"The pictures are breathtaking; the science discoveries are mind boggling. I will be sitting next to someone on an airplane and they will ask me what do I do, and I say I work for NASA and they'll say 'oh NASA,' and they think the shuttle goes to the moon and we launch from Houston but they know about Hubble."

What makes people remember Hubble when they aren't ordinarily interested in space? Hawley has wondered about that.

"Whatever it turns out to be we need to learn that lesson because we need to apply it to other things we are doing. If it's the pictures, if it's the drama is, you know you can always pick up the paper and read something new Hubble has done, maybe people think they are getting value for their tax dollars, but as far as I know we never really studied it, or asked someone who knows how to do that kind of thing to study it, and tell us what it really was about Hubble that people found so appealing."

By Jeremy Hsu16 April 2008A telescope on the far side of the moon could probe the "dark ages" of the universe while blocking out the radio-wavelength noise of Earth civilizations.

Up to one hundred thousand antennas would form the Dark Ages Lunar Interferometer (DALI), the largest telescope ever built, and allow astronomers to hear faint whispering signals from a time when no stars even existed.

"This will look at one of the most fundamental questions ever conceived, back when the universe was made up almost entirely of hydrogen and helium — no stars, no galaxies," said Kurt Weiler, senior astronomer at the U.S. Naval Research Laboratory.

The so-called dark ages of astronomy describe a half-billion year period following the Big Bang when clouds of ionized gas cooled as the universe expanded. The only faint noise came from hydrogen atoms doing spin-flips, which gives off radio-wavelength signals that astronomers can pick up on. Scientists currently estimate that the universe is about 13.7 billion years old.

"What happens is that because of the Big Bang there's a background glow," Weiler noted. "The spin-flip will absorb the glow of the older material and will give us a signature that we can see."

However, the ongoing expansion of the universe has stretched or red-shifted the hydrogen signature from just 21 centimeters to several meters. That means the signals can easily get masked by louder Earth transmissions in the same wavelength, unless astronomers find a quieter listening spot.

"The back side of the moon is the only place in the local universe shielded from manmade transmissions," Weiler told SPACE.com.

The DALI design resembles existing radio telescope arrays in the Netherlands, Australia, and New Mexico. But sending such an array to the moon requires lighter material that can save on launch costs, not to mention survive the harsh lunar conditions.

One candidate is polyimide, a plastic-like film which can act as an antenna when plated with metal. University of Colorado researchers are testing the film's durability by exposing it to harsh ultraviolet rays, as well as the extreme temperatures like that of boiling water and super-cold liquid nitrogen.

The film antennas would be rolled up and then unrolled for deployment across 30 miles (48 km) of lunar surface, arrayed in one thousand stations containing one hundred antennas each. Still, getting the entire load to the moon represents a challenge.

"Even though each antenna may weigh a few ounces, you're talking about needing at least heavy lift vehicles," Weiler noted. "They all add up fast."

The U.S. Naval Research Laboratory is sharing NASA funding with an MIT-based team working on another lunar telescope separate from DALI. Their collaboration may finally realize a dream that many astronomers had even before the first Apollo landings on the moon.

"Probing the dark ages presents the opportunity to watch the young Universe evolve," said Joseph Lazio, NRL astronomer and head of the DALI proposal. "Just as current cosmological studies have both fascinated and surprised us, I anticipate that DALI will lead both to increased understanding of the Universe and unexpected discoveries."

If your new telescope has go-to capability, all you need to do is follow the manufacturer’s instructions for initially training the instrument and then use the go-to controller to point your telescope at whatever you wish to view. Bear in mind, of course, that light pollution or other atmospheric conditions may obscure your view. Go-to technology is wonderful, but it can’t work miracles. It will point you in the right direction, but it can’t guarantee that you’ll always see what you’re looking for. If your instrument does not have a go-to controller, glance back at Chapter 1, which introduces the idea of celestial coordinates and altazimuth coordinates as well as the utility of constellations as celestial landmarks. Later chapters have more to say about finding specific objects. What you should familiarize yourself with now, however, is the finderscope affixed to the side of your telescope. Unless you have a rich-field telescope, commanding about a three- or four-degree slice of the sky, you will find it almost impossible to locate with the main telescope anything you happen to see with your naked eye. (“There’s Venus! But why can’t I find it with this #^$%@% telescope!?”) Take the time and effort to follow what your instruction manual says about adjusting the finderscope so that it can be used to locate objects quickly. This adjustment should take just a few minutes and can be done in daylight; once it’s done, it’s done (at least until you or someone else bumps the finder out of alignment). In any case, the alignment process is far less tedious and frustrating than trying to sight with your naked eye along the telescope tube and then just hoping you can finally find what you’re looking for.Another option is called a Telrad Reflex Sight. Many amateurs use one of these—an inexpensive “bullseye” on the sky. In many ways, this product is even more helpful than a finderscope.

Light pollution is the obscuring of celestial objects by artificial light sources.What do you do about it?You avoid city lights, if you can. The recent trend toward those peach-colored, highpressure, sodium-vapor and bluish metal-halide streetlights may make some people feel safer, but the lamps have also greatly increased light pollution, even in smaller urban areas.Here are some ways to reduce the effects of light pollution:

Rise above the streetlights. Set up your telescope on a hill or a safe roof. The cumulative effect of the streetlights will still blot out many of the less bright objects, especially near the horizon, but at least you won’t be trying to look up through the nearest streetlights.

Study the sky in a direction away from light sources. If your city’s downtown area is east of your location, look west rather than east.

Get out of town. Scout out some rural retreats away from the city lights but sufficiently clear of trees to allow reasonably unobstructed viewing. Local state parks are often a good option. It may be best to choose parks that offer overnight camping, since some public parks are open only from dawn to dusk. Don’t trespass!

If you get very interested in observing, you can purchase filters for your telescope that will block out a good portion of the light pollution caused by streetlights. Such filters are available from Meade Instruments, Orion, and other suppliers. Be aware, however, that these filters are most useful for astrophotography or digital imaging and are less effective if your primary imaging device is your own retina. Also, all filters block light, dimming the image you see; so small-aperture telescopes will suffer most from this side effect.

Write your local city government and encourage officials to install lowpressure, downward-facing sodium lamps. These lights have a yellowish glow and are highly energy efficient. You can get many good ideas on how to reduce light pollution from the International Dark Sky Association (find more at www. darksky.org/).

Unless you live in a nest of searchlights, there should still be enough for a beginner to see.

Once you get hooked on looking through a telescope, sooner or later you’re going to want to start recording what you see. One very enjoyable activity is to make drawings, but many serious amateurs sooner or later turn to photography. Astrophotography can be done with any good single-lens reflex (SLR) camera, the right kind of adapter to mate it with your telescope, and a sturdy tripod and mount with a tracking motor to compensate for the rotation of the earth during the long exposures are usually necessary.

As digital technology has greatly simplified and expanded the possibilities of finding objects in space, it has also simplified and expanded the field of professional as well as amateur astrophotography. We discussed how charge-coupled devices (CCDs) have largely replaced conventional photographic film for most astronomical imaging through major earth-based telescopes as well as such space-based instruments as the Hubble Space Telescope. Just as, in recent years, the cost of go-to technology has been greatly reduced, so now is digital imaging within the reach of serious amateurs. The operative word is “serious.” Meade’s Pictor 1616XTE CCD system costs more than $6,000, but the more “entry-level” Pictor 415XTE comes in at just under $2,000. And a very respectable camera from the Santa Barbara Instrument Group (SBIG) called the SBIG ST7 can be purchased (at the time of this writing) for under $3,000. It is likely that, over the years, the cost of CCD imaging will fall even further.

If you are interested in astrophotography, whether using conventional film or with CCD imaging, check out Michael A. Covington’s excellent Astrophotography for the Amateur (Cambridge University Press, 1999) or Jeffrey R. Charles’s Practical Astrophotography (Springer Verlag, 2000)

Many subsequent chapters contain advice on observing various celestial objects, but for now, having bought your telescope (and having assembled it; typically, some assembly is required), what do you do with it?In two words: Use it.You don’t need a plan, but many first-time sky watchers christen their new telescope by looking at the moon. A more original inaugural journey begins by marking out an interesting-looking piece of sky for yourself and studying it. Find what you can. Later, we’ll talk about recording what you see.Another good way to start is to go to your local library and check out Astronomy or Sky & Telescope magazine. Both of these periodicals (and their online equivalents) include a guide to the night sky in their center section every month, and you can check to see if there are any planets in the sky or which constellations are up. Also see Appendix E, “Sources for Astronomers,” for recommended guidebooks. An interesting second activity is to locate another piece of sky—one that looks almost empty—and try to find dim and distant objects. Test the limits of your new telescope and your own eyesight. Notice how many more stars you see with your finder telescope, which has a larger aperture than your eye, and then notice how many stars you see in your main telescope.

Tuesday, April 8, 2008

Books on amateur astronomy used to supply only two important pieces of advice about tripods and mounts.First: Don’t cheap out. Invest in something sturdy and steady.Second: Choose between an altazimuth mount and an equatorial mount. The simple altazimuth mount is adjustable on two axes: up and down (altitude) and left and right (movement parallel to the horizon, or azimuth). There is nothing automatic about most altazimuth mounts. If you are trying to follow an object, you must continually adjust both the altitude and the azimuth. The alternative equatorial mount is aligned with the earth’s rotational axis and, therefore, may be made to follow a celestial object by adjusting one axis only (to counteract the rotation of the earth).

These two pieces of advice used to be quite sufficient. In the late 1990s, however, popular manufactures started selling even some entry-level telescopes with go-to computer controllers that drive servo-motors built into the telescope mount. The handheld go-to controller stores a database of the locations of thousands of celestial objects. Select a object or punch it its coordinates, and (if properly trained and aligned) the telescope’s servos will point the telescope at your target object.

In addition to servo motors for go-to capability, equatorial and altazimuth mounts typically include a clock drive that synchronizes the telescope with the earth’s rotation so that a given object can be followed—“tracked”—without your having continually to re-aim the telescope.Go-to capability can work on telescopes that have either altazimuth or equatorial mounts. For example, the go-to features on the Meade ETX 90EC telescope can be used in either equatorial or altazimuth mode. One just has to be careful that the computer has been informed of your choice (usually accomplished on the setup menu). The amazing thing is that go-to technology has become sufficiently affordable to be included in even entry-level telescopes. This feature has truly revolutionized amateur astronomy, greatly broadening its appeal. Keep in mind that the “go-to” hand paddle must typically be purchased as an accessory, and will cost several hundred dollars itself. If this capability is important to you, you should buy a telescope that can be updated at a later time.

During the 1970s, an avid amateur astronomer named John Dobson began building large, standard Newtonian reflectors (10-inch mirrors were typical) and cutting costs by mounting them not on elaborate and expensive equatorial mounts but on inexpensive altazimuth mounts. Dollars were invested in optics and aperture—lightgathering ability—rather than in fancy mounting hardware and clock drives to aid in tracking objects. The result was a powerful reflecting telescope with a wide field of view.

Very nice Dobsonians can be purchased in the $300 to $1,000 range, or you could see if your local amateur astronomy club offers workshops in making your own telescope. Many astronomy club members make their own Dobsonians. Is there a Dobsonian downside? Some users find the simple altazimuth mount—which lacks the ability to track objects—too limiting.

Like the Schmidt-Cassegrain telescopes, the Maksutov-Cassegrain is a catadioptric design; however, these newer instruments optimize imaging performance by combining a special spherical meniscus (concave) lens with two mirrors. The secondary mirror multiplies the focal length of the telescope. The combined effect of the concave lens, the aspherical primary mirror, and the convex secondary mirror produces a telescope that is almost as well suited to lunar and planetary observation as a refractor, yet it has many of the reflector’s advantages for deep-space viewing. These qualities are similar to the conventional Schmidt-Cassegrain design, but the Maksutov-Cassegrain variation tends to yield images of greater contrast than one gets from telescopes of the earlier design.

A 7-inch Maksutov is significantly more expensive than an 8-inch Schmidt-Cassegrain; however, Meade has marketed for some years now two extremely popular small Maksutov models, the ETX-90EC and ETX-125EC (90 mm and 125 mm, respectively), which trade aperture for price. The 90-mm model can be purchased for under $500, and the 125-mm model for less than $900.Diagram of a Maksutov-Cassegrain telescope. Light enters the concave lens at the right and is reflected by the aspheric primary mirror at the left, which sends it back to the spherical secondary mirror on the right. This, in turn, focuses the image on the focal plane on the left.

Friday, April 4, 2008

Also called a catadioptric telescope, the Schmidt-Cassegrain design combines mirrors and lenses. Telescopes of this design are an increasingly popular choice for serious amateurs and introductory astronomy classes. The light passes through a corrector lens before it strikes the primary mirror, which reflects it to a secondary mirror. Since light bounces down the tube an extra time, the focal length of the telescope is effectively doubled, belying the very compact—wide but short and stubby—look of the instrument. A long effective focal length means that these telescopes can have a high magnification (remember that magnification is the ratio of objective focal length to eyepiece focal length) without a cumbersome long tube. Schmidt-Cassegrain telescopes are elegant instruments that offer some of the compactness of rich-field instruments but are much more powerful. The catch?

Diagram of a Schmidt-Cassegrain, or catadioptric, telescope. Light enters from the left and is focused by the primary mirror at the back of the telescope. Then it is refocused by a secondary mirror and sent out through an opening in the primary mirror to an eyepiece at the rear of the telescope.

These are usually more expensive amateur instruments, typically priced from $900 to much, much more, depending on aperture size and features. The portability of the Schmidt-Cassegrain design is a very big plus—not just because a compact telescope is easier to transport, but also because it is easier to keep a small scope stable during use.

Worth investigating is a relatively new category of telescope. Ultra compact and reasonably priced, rich-field reflectors are typically handheld with a Newtonian focus. What they have in common is a short-tube design that offers low degrees of magnification but a bright, wide field of view (typically a few degrees). They range in price from about $250 to $400 and can weigh as little as 4 or 5 pounds. Highly portable and relatively rugged, these telescopes nevertheless have the disadvantage that, since they are handheld, they do not track with objects in the sky, and are only as steady as you are. The great advantages of these telescopes, besides price, are their portability and the brightness of the image they deliver.

RefractorMost astronomers agree that a good refractor is the instrument of choice for viewing the moon and the planets. Typically, the refractor’s field is narrow, which enhances the contrast offered by good optics and brings out the details of such things as the lunar surface and planetary detail.Refractors, however, are not the best choice for deep-sky work—looking at dim galaxies, for example. They are great for bright objects, but a refracting telescope with the same light-collecting ability of a decent reflecting telescope would be prohibitively expensive.Some of the cheapest, mass-market telescopes are refractors, but most of these will perform poorly. Most good refractors are long, heavy, and expensive—although the recently introduced Meade ETX-60AT and ETX-70AT are compact yet high-quality entry-level instruments. The disadvantage of expense is obvious, as is that of weight: You’ll be discouraged from taking the telescope with you on trips to the dark skies of the country. Length poses a less obvious problem. The longer the tube, the less inherently steady the telescope. A large refractor requires a very firm mount and tripod.

ReflectorsTraditionally, the Newtonian reflector has been the most popular telescope with experienced amateurs, although, in recent years, affordable Schmidt-Cassegrain and Maksutov-Cassegrain instruments have found increasing favor. Generally, a reflector gives you more aperture—and thus more light—for your dollar than a refractor, and the reflector’s mirror is not subject to chromatic aberration (the differences in the ways various colors, especially red and blue, are focused), which all but the most expensive refractor lenses suffer from. Although reflectors may be large, they are generally lighter than refractors; however, they tend not to be as robust, and unlike a good refracting telescope, they do require at least some minimal maintenance to realign optical components occasionally.